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- Author or Editor: Andreas Reuter x
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Abstract
The potential of the 2D-Grey optical array probe (OAP) (with 10-μm resolution) to determine cloud microphysical properties is studied. Systematic test measurements with a spinning glass disk with sample spots of various sizes between 50 and 500 μm in diameter were conducted. These measurements show that the particle image diameter increases considerably if the particle crosses the illuminating laser beam at increasing distance from the object plane. Eventually, shadow images of the smaller spots lose even their circular image shape and appear fragmented. A method is proposed to improve the estimation of the nominal particle size of droplets from the recorded image by exploiting the four available shadow (grey) levels. Laboratory tests show that spherical particles from 50 to 500 μm in diameter can be properly sized with an rms uncertainty of less than 6%. After discussion of the concept of depth of field in OAPs, a definition for the 2D-Grey probe is presented that is consistent with the standard definition for the 2D-C probe. The authors’ measurements show the depth of field of the 2D-Grey probe to be three times larger than the value conventionally assumed for the 2D-C probe for which similar corrections have been recently discussed in the literature. Finally, the impact of these findings on particle size distribution for in situ measurements is discussed.
Abstract
The potential of the 2D-Grey optical array probe (OAP) (with 10-μm resolution) to determine cloud microphysical properties is studied. Systematic test measurements with a spinning glass disk with sample spots of various sizes between 50 and 500 μm in diameter were conducted. These measurements show that the particle image diameter increases considerably if the particle crosses the illuminating laser beam at increasing distance from the object plane. Eventually, shadow images of the smaller spots lose even their circular image shape and appear fragmented. A method is proposed to improve the estimation of the nominal particle size of droplets from the recorded image by exploiting the four available shadow (grey) levels. Laboratory tests show that spherical particles from 50 to 500 μm in diameter can be properly sized with an rms uncertainty of less than 6%. After discussion of the concept of depth of field in OAPs, a definition for the 2D-Grey probe is presented that is consistent with the standard definition for the 2D-C probe. The authors’ measurements show the depth of field of the 2D-Grey probe to be three times larger than the value conventionally assumed for the 2D-C probe for which similar corrections have been recently discussed in the literature. Finally, the impact of these findings on particle size distribution for in situ measurements is discussed.
Abstract
Laboratory measurements of the response of the Particle Measuring Systems, Inc., 2DC probe have been conducted to characterize counting and sizing errors of the probe for spherical particles. Measurements of the shadow threshold intensity of a Meteorological Service of Canada (MSC) 2DC probe varied from approximately 30% to 51%, depending on the photodiode, and averaged 46% for the central 16 photodiodes. Depth-of-field and sizing measurements are quite sensitive to this threshold, which is nominally considered as 50% for the 2DC probe. Response times also varied significantly, from 0.44 to 0.90 μs. Measurements of the depth of field for known particle sizes at low velocity agreed well with published calculations at zero velocity. For particles smaller than 100 μm, the depth of field decreased significantly with increasing airspeed due to the nonzero response time of the sensing photodiodes. The average particle size also decreased with increasing airspeed but did so in such a manner as to counteract oversizing due to out-of-focus images. At 100 m s−1, the average measured sizing error of a 100-μm particle was close to negligible, rising to approximately 5% at 500 μm. The application of measured depth-of-field values and sizing calibrations at specific sizes to improve 2DC size distribution accuracy is nontrivial because measurement errors cause particles to be redistributed to other sizes in a complicated manner. However, when hypothetical true particle distributions were redistributed according to a distortion matrix approximated by the results of this study, the average error of uncorrected size distributions measured by the MSC 2DC probe, expressed as a sizing error, was found to be ±10% for particles larger than 125 μm. Although these results are not strictly transferable to other 2DC probes, the methods described can be used to derive similar results for other probes.
Abstract
Laboratory measurements of the response of the Particle Measuring Systems, Inc., 2DC probe have been conducted to characterize counting and sizing errors of the probe for spherical particles. Measurements of the shadow threshold intensity of a Meteorological Service of Canada (MSC) 2DC probe varied from approximately 30% to 51%, depending on the photodiode, and averaged 46% for the central 16 photodiodes. Depth-of-field and sizing measurements are quite sensitive to this threshold, which is nominally considered as 50% for the 2DC probe. Response times also varied significantly, from 0.44 to 0.90 μs. Measurements of the depth of field for known particle sizes at low velocity agreed well with published calculations at zero velocity. For particles smaller than 100 μm, the depth of field decreased significantly with increasing airspeed due to the nonzero response time of the sensing photodiodes. The average particle size also decreased with increasing airspeed but did so in such a manner as to counteract oversizing due to out-of-focus images. At 100 m s−1, the average measured sizing error of a 100-μm particle was close to negligible, rising to approximately 5% at 500 μm. The application of measured depth-of-field values and sizing calibrations at specific sizes to improve 2DC size distribution accuracy is nontrivial because measurement errors cause particles to be redistributed to other sizes in a complicated manner. However, when hypothetical true particle distributions were redistributed according to a distortion matrix approximated by the results of this study, the average error of uncorrected size distributions measured by the MSC 2DC probe, expressed as a sizing error, was found to be ±10% for particles larger than 125 μm. Although these results are not strictly transferable to other 2DC probes, the methods described can be used to derive similar results for other probes.
